NFV-Style DDoS mitigation using Snabb Switch

My employer arranged for a hack day last month. It meant anyone participating was free to hack on anything they wanted and at the end of the day we got to present our work during a 2 minute flash presentation to our colleagues as well as a number of students from KTH’s (Royal institute of technology) computer science program.

Together with my colleague, Johan Törnhult, we set out to build a simple DDoS mitigation application. I had previously looked at Snabb Switch and have followed the project, albeit from a distance, for quite some time. This was my chance to get acquainted with it and try out my programming skill set at Lua, a language I had never touched or even seen before.

In its essence, Snabb Switch is a framework to offer developers or network engineers with some programming skills (like me) to build their own packet forwarding logic. Code is packaged into “apps” and multiple apps can be linked together in a graph. You can’t call Snabb a router or a switch because out of the box it neither routes or switches packets. With a minimal configuration you can have it forward packets (verbatim) from one port to another but as soon as you want to do something more advanced you would need an app.

Snabb comes with a number of apps, ranging from very small apps to replicate incoming packets over multiple outputs or read a pcap file and send over the network to more complete implementations like a VPWS app to build L2VPNs over an IP network.

There is definitely some room for improvement as far as the documentation for Snabb goes. It’s a fast moving project, still in its relatively early life, which means some documentation is out of date while most of the documentation hasn’t been written yet. Mostly by reading the code of other apps and some of the existing documentation I managed to forward my first packet through Snabb.

My intention was to implement a source host blocking mechanism that would block any host sending over a specified amount of packets or bytes per second and a few hours after my first unsteady steps I had my first working implementation. Performance was horrendous and it wouldn’t recognize IPv6 packets correctly instead assuming everything to be IPv4. I did manage to find a library to apply BPF filters (the same you use to specify a filter with tcpdump) on a packet which I used to define rules. A rule has a traffic pattern to match and a pps and bps threshold. Once a source host exceeds the threshold for a rule it will be completely blocked for a certain amount of time.

The test machine used managed to push some 300Kpps of packets and over the next day or two I managed to tweak a few things to raise performance. One really comes to understand how crucial it is for performance to keep down the amount of code executed and the amount of data copied between various places. My first change was to use a library to parse the headers of the packet, first looking at the ethertype of the Ethernet header to determine if it was an IPv4 or IPv6 packet and after that to extract the source IP address from the packet. While I achieved support for IPv4 and IPv6, this parsing library proved to be fairly slow and instead I moved to just extracting the few bits that represent the ethertype and after that got the source IP address. Extracting the address used ntop (network to presentation) to convert 4 or 16 bytes to a string representation of the IP address. Again this is slow and simply having an IPv4 address represented as a uint32_t using Lua FFI (allowing C data structures within Lua) proved to be an order of magnitude faster.

I’m now up to around 5Mpps in real packet forwarding on a single core, when the majority of packets are being blocked. A selftest on my laptop, which is a fair bit slower than the “real” test machine, does 10Mpps which is somewhat surprising. Instead of receiving packets on a NIC, processing using my logic and sending to the next NIC (or dropping) it will read packets from a pcap file and continuously loop those packets into my app. I wouldn’t have guessed the overhead surrounding handling real NICs would be that large.

Arbor implements something very similar in their TMS and Pravail series of products, which they call “Zombie Detection”. 10Gbps/5Mpps of performance is in excess of $125k from Arbor while my x86 PC with 4 cores can do twice that for just under $2500 - that’s two orders of magnitude cheaper, no surprise NFV is gaining traction. I know my comparison isn’t fair but it just comes to show what can be done and what the future might hold.

You can find the code for the DDoS source blocker at GitHub/plajjan/snabbswitch. Reach out to me on Twitter if you want to ask me anything about it or discuss how to take it further - I would love someone to collaborate with :)

If you are interested in writing packet forwarding logic and haven’t looked at Snabb Switch yet, I recommend you to do so - it is surprisingly easy to get going with!

Written on December 13, 2014